This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device characterized by a structure composed of both a bus line and an active element by which each pixel electrode placed in the form of a matrix is switched on and off. Furthermore, this invention relates to the manufacturing method of the above-described liquid crystal display device.
Conventionally, a liquid crystal display device comprises a pixel electrode placed in the form of a matrix, a bus line and an active element for switching the pixel electrode.
Recently, there has been strong demand for a liquid crystal display device with a wide display screen composed of fine dots. In order to meet this requirement, the bus line necessarily becomes longer. Therefore, it is necessary to make the bus line from materials with low electrical resistance. Accordingly, it is necessary to form the thick layer bus line from materials with high conductivity.
Active elements are assembled in the form of a laminated structure. In order to increase the reliability of the active elements by preventing them from being damaged, it is preferable to form a laminated structure by covering the active elements on a condition where a level difference is eliminated as far as possible. For that reason, it is necessary to make the electrode layer on the transparent substrate side thin.
However, from the viewpoint of simplifying the manufacturing process, it is preferable that a portion of the bus line is used as the electrode of the active element on the transparent substrate side. In other words, a portion of the bus line which was made thick, a portion of the bus line which branches from the bus line to the active element side, or a position of the bus line itself is used as the active element electrode on the transparent substrate side. It is preferable that a branch of the bus line is used as the gate in the case of the reverse stagger TFT (thin-film transistor), and a branch of the bus line is used as the source in the case of the normal stagger TFT. Furthermore, it is preferable that a branch of the bus line is used as one of the electrodes in the case of MIM (metal insulator metal), and a branch of the bus line is used as the electrode to form a Schottky barrier in the case of a back to back Schottky diode.
However, the following are the problems caused when a portion of the bus line is used as the electrode of the active elements on the transparent substrate side.
(1) When a thick layer is used as the bus line in order to reduce electrical resistance to meet the requirements of a large and fine display unit, the electrode of the active elements on the transparent substrate side, which is a portion of the bus line, becomes a thick layer. As a result, a level difference on a covered condition in the laminated structure of the active elements becomes too large and then defects such as a short-circuit and a deterioration in withstanding voltage may increase. This leads to a decrease in the reliability of the active element. PA1 (2) On the other hand, when a thin layer is used as the electrode on the transparent substrate side of the active elements is made a thin layer in order to accomplish no level difference on a covered condition, there is no alternative but to reduce the layer thickness of the bus line itself. As a result, the electrical resistance of the bus line increases and it becomes difficult to obtain a wide display screen composed of fine dots. PA1 (1) Since resist work must be conducted many times, the manufacturing process becomes complicated. To go into more detail, the following resist works are needed. One is the resist work to remain a narrow resistant layer on the bus line for the gate. The other is the resist work to remove a prescribed amount from both the bare upper portion of the bus line for the gate and the upper portion of the gate electrode by the anisotropic dry etching method. PA1 (2) Since a narrow resistant layer is left on the bus line for the gate by over-exposing, controlling the over-exposure is difficult. Accordingly, the shape of the bus line with a stepped formation, is apt to fluctuate and the reliability is decreased. PA1 (3) Since a prescribed amount is removed from both the bare upper portion of the bus line for the gate and the upper portion of the gate electrode by the anisotropic dry etching method in order to obtain a thin layer, it is difficult to obtain a prescribed thin layer and the characteristics of the reverse stagger TFT fluctuates. As a result, the reliability is decreased. PA1 (4) As the bus line for the gate and the gate electrode are combined into one unit, they must be made from the same material. For that reason, the bus line for the gate and the gate electrode can not be made from adequate materials. PA1 (1) It is an object of the invention to provide a liquid crystal display device in which a wide display screen composed of fine dots is installed by reducing the electrical resistance of the bus line, and in which an active element of the laminated structure on the covered condition with no level difference is formed by making the electrode on the transparent side of the active elements. PA1 (2) It is another object of the invention to provide a liquid crystal display device manufacturing method by which a reliable bus line and active element can be formed with less resist work and furthermore by a simple process, and by which the electrical resistance of the bus line can be reduced and the electrode of the active element on the transparent substrate side can be made thin. PA1 Process (1): A transparent conductive layer is formed on one side of a transparent substrate. Furthermore, the first wiring layer is laid on the transparent conductive layer. PA1 Process (2): Patterning is conducted on the laminated layer consisting of the transparent conductive layer and the first wiring layer formed in Process (1) according to the pattern to form the bus line and the pixel electrode. PA1 Process (3): The second wiring layer is laid on the laminated layer consisting of the transparent conductive layer and the first wiring layer on which the patterning was conducted in Process (2) and on the transparent substrate. PA1 Process (4): Patterning is conducted on the second wiring layer formed in Process (3) according to the pattern to form the bus line and the electrode of the active element. PA1 Process (5): The unnecessary portion of the first wiring layer is removed according to the same pattern as that in Process (4).
According to the circumstances described above, the following method was disclosed recently in Japanese Patent Publication Open to Public Inspection No. 070281/1988 and No. 070282/1988. In a liquid crystal display device in which reverse stagger TFT is used as an active element, a method of providing a bus line with a stepped formation and a gate electrode of a thin layer, comprises the steps of; forming on a transparent substrate a bus line for a gate and a gate electrode consisting of a branch of the bus line for the gate; forming a resistant layer on the transparent substrate including them; remaining a narrow resistant layer on the bus line for the gate by over-exposing the reverse side of the transparent substrate with a mask on the bus line and the gate electrode for self-alignment; and removing a prescribed amount from both the bare upper portion of the bus line for the gate and the upper portion of the gate electrode by the anisotropic dry etching method wherein the resistant layer is used as the mask.
According to the method described above, the thickness of the main portion of the bus line for the gate can be increased and the electrical resistance of the bus line can be reduced. Furthermore, the layer thickness of the gate electrode can be reduced and reverse stagger TFT can be made in the laminated structure on the covered condition with no difference in level.
According to the method disclosed in the publication, there are several problems which will be described as follows.